This invention relates to an electric shutter operating circuit for a photograhic camera capable of responding to light of extremely low intensity.
As diagrammatically shown in FIG. 1, a prior art device responsive to light of extremely low intensity for automatically setting the exposure time of a photographic camera comprises a light receiving element CdS such as a CdS photoconductor, a capacitor C connected in series with the light receiving element, a resistor R connected in parallel with the light receiving element and a trigger circuit TR. This device is constructed in a manner such that the brightness of the object tends to establish either the relation R&gt;&gt;R.sub.c or the relationship R&lt;&lt;R.sub.c where R represents the resistance value of resistor R, and R.sub.c that of the photoconductor CdS. Under brightness conditions where R&gt;&gt;R.sub.c, the shutter time is determined by R.sub.c and capacitor C whereas at another brightness in which R&lt;&lt;R.sub.c the shutter time is determined by the value of R. For this reason, even at an extremely low brightness condition wherein R.sub.c is infinite, the time constant of the circuit consisting of R and C functions to close the shutter. However, under brightness conditions where the resistance values of R and R.sub.c are substantially equal, their combined value reduces to one half so that the exposure value becomes less than 1 EV, thus causing an exposure error. Further, under brightness conditions where R.sub.c &gt;R, the parallel resistor R further increases the exposure error.